Non-contact chemical mechanical polishing wafer edge control apparatus and method

ABSTRACT

Methods and apparatus for controlling a material removal rate at an edge of a wafer during a chemical mechanical polishing (CMP) process are disclosed. According to one aspect of the present invention, a CMP apparatus includes a wafer, a polishing pad to polish a surface of the wafer, a polishing pad structure to rotate the polishing pad over the surface of the wafer, and a wafer chuck to support the wafer. The wafer chuck directly supports a first portion of the wafer that is in physical contact with the wafer chuck and indirectly supports a second portion of the wafer that is not in physical contact with the wafer chuck. The second portion of the wafer is supported by the wafer chuck using a bearing surface arranged between the second portion of the wafer and the wafer chuck.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to chemical mechanical polishing systems. More particularly, the present invention relates to an arrangement that allows the removal of material from an edge of a wafer to be controlled.

2. Description of the Related Art

Ensuring the planarity of the surface of a semiconductor wafer is crucial if the integrity of photolithography processes performed on the semiconductor wafer is to be maintained at a high level. That is, it is important that the surface of a semiconductor wafer be planar in order to meet the requirements of photolithography processes. By way of example, the planarity of the surface of a semiconductor wafer is critical to photolithography processes as the depth of focus of photolithography processes may not be adequate for surfaces which do not have a consistent height.

A chemical mechanical polishing (CMP) process is often used to planarize the surface of a semiconductor wafer. CMP is effective in improving the global planarity of the surface of a semiconductor wafer. The assurance of planarity is crucial to the lithography process as the depth of focus of the lithography process is often inadequate for surfaces which do not have a consistent height.

CMP processes generally utilize a polishing pad made from a synthetic fabric and a polishing slurry which includes pH-balanced chemicals, such as sodium hydroxide, and silicon dioxide particles. A semiconductor wafer is mounted on a polishing fixture such that the wafer is pressed against the polishing pad under pressure. The fixture then rotates and translates the wafer relative to the polishing pad. The polishing slurry assists in the actual polishing of the wafer. Abrasive forces are created by the motion of a wafer against a polishing pad and cause material to be abraded away from the surface of the wafer. While the pH of the polishing slurry controls chemical reactions such as the oxidation of the chemicals which comprise an insulating layer of the wafer, the size of the silicon dioxide particles in the polishing slurry controls the physical abrasion of surface of the wafer. The polishing of the wafer is accomplished when abrasive forces enable the silicon dioxide particles to abrade away the oxidized chemicals. Often, different layers of the wafer may be thinned to a desired thickness through CMP.

With reference to FIGS. 1A and 1B, one conventional CMP system will be described. FIG. 1A is a diagrammatic top-view representation of a CMP polishing system 100, and FIG. 1B is a diagrammatic side-view representation of CMP polishing system 100. A CMP polishing system 100 includes a polishing pad or platen 104 attached to an actuator assembly (not shown) at an annulus 108. As polishing pad 104 rotates about a z-direction 110 while making contact with a wafer 114 that is being polished. Wafer 114 also rotates about z-direction 110.

Wafer 114 is typically secured or held in place by a retaining ring during CMP (not shown). A retaining ring holds a wafer near the edge of the wafer such that an entire surface of the wafer may substantially always be in contact with a polishing pad. FIG. 2A is a diagrammatic cross-sectional side-view representation of a conventional CMP system in which a retaining ring is used to hold a wafer. In a CMP system 200, a wafer 214 is held by a retaining ring 220 such that when a membrane 224 applies pressure to wafer 214, wafer 214 is polished by polishing pad 204. Retaining ring 220, which contacts the side edge of wafer 214, is arranged to effectively “pre-deflect” polishing pad 204, as shown in FIG. 2B. Retaining ring 220 contacts polishing pad 204 and deflects polishing pad 204 such that a surface 228 of wafer 214 that is being polished is substantially always in contact with polishing pad 204 during CMP.

Though retaining ring 220 is generally effective in ensuring that an entire surface 228, including edges of surface 228, may be relatively evenly polished, the use of retaining ring 220 involves critical tolerances. By way of example, tolerances associated with wafer 214 and retaining ring 220 are typically such that any mismatch of tolerances may cause wafer 214 not to be securely held by retaining ring 220.

Some CMP systems are such that an entire surface of a wafer that is to be polished is not always in contact with a polishing pad during CMP. By way of example, a CMP system may utilize a polishing pad that has a smaller diameter than a wafer that is to be polished. FIG. 3A is a diagrammatic top-view representation of a CMP system that utilizes a polishing pad that has a smaller diameter than a wafer. A CMP system 300 includes a polishing pad or platen 304 that is arranged to polish a wafer 308. Both polishing pad 304 and wafer 308 are arranged to rotate about a z-direction 318, as indicated by arrow 324 and arrow 328, respectively. Wafer 308 is supported on a supporting table 334 or wafer chuck which allows wafer 308 to rotate, as shown in FIG. 3B. Polishing pad 304 is further arranged to oscillate, as indicated at 332, such that polishing pad 304 moves on and off of wafer 308.

CMP system 300 is such that an entire surface of wafer 308 is not always in contact with polishing pad 304, i.e., polishing pad 304 transitions on and off of wafer 308 during CMP. When polishing pad 304 transitions on or off of wafer 308, the edge of wafer 308 may deflect. FIG. 4A is a diagrammatic cross-sectional side-view representation of a polishing pad transitioning on to, or off of, a wafer. A polishing pad 404, which is a part of a polishing head 402, is partially in contact with a wafer 408 and partially out of contact with wafer 408. In addition to polishing pad 404, polishing head 402 includes a firm backing 410 and a compressible layer 406 which may be a urethane layer.

Wafer 408 is supported on a wafer chuck 436. Typically, wafer chuck 436 uses a vacuum to secure wafer 408 thereon. An edge of wafer 408 are arranged not to be in physical contact with wafer chuck 436 such that edge beads, or material deposits in proximity to the bottom edge of wafer 408, do not affect the security or deflection of wafer 408 relative to wafer chuck 436. Further, the lack of physical contact between wafer chuck 436 and the bottom edge of wafer 408 prevents slurry used in CMP from entering into the vacuum system of wafer chuck 436, and also provides some tolerances that allow the positioning of wafer 408 relative to wafer chuck 436 to be adjusted.

A fluid, as for example de-ionized water, may be provided through an opening or openings 440 in wafer chuck 436 to flush out any debris that is present between the bottom edge of wafer 408 and wafer chuck 436. The fluid may also keep the bottom edge of wafer 408 clean by washing away debris that may be present on the bottom edge of wafer 408.

When polishing pad 404 is partially in contact with wafer 408 near an edge of wafer 408, the edge of wafer 408 may deflect as a result of the pressure applied by polishing pad 404. As shown in FIG. 4B, an edge 450, e.g., a bottom edge, of wafer 408 deflects in the presence of pressure 460 applied to wafer 408 such that edge 450 is deflected to a position 450′. That is, wafer 408 effectively becomes a cantilever in the vicinity of edge 450 when a polishing pad such as polishing pad 404 of FIG. 4A applies a substantially uniform pressure 460 to wafer 408 at or near edge 450. As a clearance H 480 between wafer chuck 436 and wafer 408 is between approximately 70 micrometers (□m) and approximately 100 □m, even in position 450′, edge 450 is not likely to come into physical contact with wafer chuck 436.

The deflection of edge 450 may cause the removal rate of material near edge 450 to be changed relative to the removal rate of material on other portions of wafer 408, e.g., portions of wafer 408 not near edge 450. As edge 450 deflects, the pressure applied to wafer 408 at and near edge 450 changes, thereby causing the removal rate of material to change. In other words, edge 450 deflects under a dynamic pressure load from polishing pad 404, and the deflection of edge 450 causes the removal rate of material near edge 450 to be difficult to control. When the removal rate of material near edge 450 is different from the removal rate of material on other portions of wafer 408, the uniformity with which material is removed from wafer 408 is compromised. As a result, the integrity of wafer 408 may be compromised.

Therefore, what is needed is a method and an apparatus that allows a wafer to be supported such that an edge of the wafer is not in physical contact with a wafer chuck and is not subjected to significant deflection. That is, what is desired is a system that enables the polishing pressure associated with an edge of a wafer to effectively be controlled.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to chemical mechanical polishing (CMP) apparatus with a wafer chuck which supports an edge periphery of a wafer using a boundary layer and enables a material removal rate profile to be controlled. According to one aspect of the present invention, a CMP apparatus includes a wafer, a polishing pad to polish a surface of the wafer, a polishing pad structure to rotate the polishing pad over the surface of the wafer, and a wafer chuck to support the wafer. The wafer chuck directly supports a first portion of the wafer that is in physical contact with the wafer chuck and indirectly supports a second portion of the wafer that is not in physical contact with the wafer chuck. The second portion of the wafer is supported by the wafer chuck using a bearing surface arranged between the second portion of the wafer and the wafer chuck.

In one embodiment, a first supply of the CMP apparatus provides the wafer chuck with a supply such that a boundary layer is created between an edge surface of the wafer chuck and the second portion of the wafer. The boundary layer forms the bearing surface. In such an embodiment the first supply may be an air supply and the bearing surface may be an air bearing surface. Alternatively, the first supply may supply a fluid, and the bearing surface may be a fluid bearing surface.

Supporting the edge of the wafer indirectly using a bearing surface substantially without physical contact between a wafer chuck and the wafer prevents slurry from entering into the wafer chuck and allows for tolerances to be maintained. At the same time, the amount of deflection of the edge of the wafer is substantially minimized. The removal rate of material from the edge of the wafer may effectively be controlled by modulating or otherwise adjusting the amount of pressure distributed along the bottom edge periphery of the wafer. Hence, a CMP process may occur accurately and efficiently.

According to another aspect of the present invention, a method for controlling a removal rate of material from a top surface of a wafer supported on a wafer chuck during CMP includes applying a polishing pressure to the top surface of the wafer and applying a support pressure to an edge periphery of a bottom surface of the wafer. The support pressure is applied through a gap between an edge surface of the wafer chuck and the edge periphery of the bottom surface of the wafer that is not in physical contact with the wafer chuck.

In one embodiment, the support pressure is in the range of between approximately 0.3 Mega Pascal (MPa) and approximately 0.6 MPa. In another embodiment, applying the support pressure to the edge periphery of the bottom surface of the wafer includes distributing an air or fluid pressure on the edge periphery of the bottom surface of the wafer. The air or fluid pressure acts as an air or fluid bearing in the gap.

These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1A is a diagrammatic top-view representation of a first chemical mechanical polishing (CMP) system.

FIG. 1B is a diagrammatic side-view representation of a first CMP system, i.e., CMP system 100 of FIG. 1A.

FIG. 2A is a diagrammatic cross-sectional side-view representation of a first CMP system in which a retaining ring is used to hold a wafer.

FIG. 2B is a diagrammatic cross-sectional side-view representation of a first CMP system, i.e., CMP system 200 of FIG. 2A, which shows a retaining ring causing a polishing pad to deflect.

FIG. 3A is a diagrammatic top-view representation of a second CMP system.

FIG. 3B is a diagrammatic perspective representation of a second CMP system, i.e., CMP system 300 of FIG. 3A.

FIG. 4A is a diagrammatic cross-sectional side-view representation of a polishing pad transitioning on to, or off of, a wafer.

FIG. 4B is a diagrammatic cross-sectional side-view representation of a wafer undergoing deflection in the presence of applied pressure.

FIG. 5A is a diagrammatic cross-sectional side-view representation of a polishing pad transitioning on to, or off of, a wafer supported by a wafer chuck with a bearing surface in accordance with an embodiment of the present invention.

FIG. 5B is a diagrammatic cross-sectional side-view representation of a portion of a wafer supported by a wafer chuck with a bearing surface, e.g., wafer 508 and wafer chuck 536 of FIG. 5A, in accordance with an embodiment of the present invention.

FIG. 5C is a diagrammatic cross-sectional side-view representation of a wafer supported by a wafer chuck with a bearing surface, e.g., wafer 508 and wafer chuck 536 of FIGS. 5A and 5B, as subjected to pressure forces in accordance with an embodiment of the present invention.

FIG. 6 is a process flow diagram which illustrates one method of performing CMP using a system which allows for wafer edge pressure control in accordance with an embodiment of the present invention.

FIG. 7 is a block diagram representation of a CMP system in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For a chemical mechanical polishing (CMP) system in which there is a gap between a wafer surface and a wafer chuck around the edge of the wafer, issues associated with deflection of the edge of the wafer during a polishing process may compromise the integrity of the polishing process. When the wafer deflects, contact between the deflected portion of the wafer and a polishing pad effectively changes, thereby causing the removal rate of material at the edge of the wafer to differ from the removal rate of material at non-edge areas of the wafer.

By supporting the edge of the wafer substantially without physical contact between a device the holds the wafer, e.g., a wafer chuck, and the wafer, the benefits of having a gap between the wafer surface and the device are realized, while the amount of deflection of the edge of the wafer is substantially minimized. Further, non-contact edge support allows the removal rate of material from the edge of the wafer to effectively be controlled. In other words, a material removal rate profile associated with a CMP process may be controlled. The edge of a wafer may be supported on an air bearing or a fluid bearing such that is arranged under the edge periphery of the wafer. Air or fluid pressure may be distributed under the edge periphery such that the pressure distribution acts as an air bearing or as a fluid bearing, respectively.

An air or fluid, e.g., water, bearing may effectively be created by maintaining a relatively small distance between an edge surface of a wafer chuck and a bottom edge of a wafer. The relatively small distance allows an air or fluid boundary layer to be created that effectively supports the edge periphery of the wafer. FIG. 5A is a diagrammatic cross-sectional side-view representation of a polishing pad transitioning on to, or off of, a wafer supported by a wafer chuck with a bearing surface in accordance with an embodiment of the present invention. A wafer 508 is supported on a wafer chuck 536 which typically uses vacuum pressure to hold wafer 508 in place. Wafer chuck 536 includes an opening or openings 540 though which air or fluid, e.g., de-ionized water, may flow. In the described embodiment, the air or fluid cooperates with an outside edge surface 550 of wafer chuck 536 to create a bearing surface that supports a bottom edge periphery of wafer 508. The air or the fluid also flushes out debris from beneath wafer 508 such that the bottom edge periphery of wafer 508 may remain relatively debris-free.

As a bearing surface between outside edge surface 550 and wafer 508 enables air or fluid to be distributed around the bottom edge periphery of wafer 508 to support wafer 508, when a polishing pad assembly or head 502 transitions on or off of an edge of wafer 508, the edge of wafer 508 may be prevented from deflecting significantly in a z-direction 554. A polishing pad 504, which along with a backing 510 and a compressible layer 506 is a part of polishing pad assembly 402, is partially in contact with wafer 508 and partially out of contact with wafer 508. Although the edge periphery of wafer 508 is not directly physically supported by wafer chuck 536, the bearing surface over outside edge surface 550 provides a pressure distribution of air or fluid that supports wafer 508 about the edge periphery. Hence, when polishing pad assembly 402 transitions on or off of wafer 508, the edge of wafer 508 over which the transition occurs does not deflect significantly.

FIG. 5B is more detailed representation of wafer 508 and wafer chuck 536 in accordance with an embodiment of the present invention. As previously mentioned, a relatively small gap Z1 568 is maintained between outside edge surface 550 and a bottom edge of wafer 508. While the size of gap Z1 568 may vary, in one embodiment, gap Z1 568 is approximately 10 micrometers (□m). In general, gap Z1 568 is less than approximately 70 □m. The air or the fluid that is provided through opening 540 may be arranged to provide pressure under the edge periphery of wafer 508, or the portion of wafer between an outside edge of wafer 508 and a portion of wafer 508 that is supported by a land 558 of wafer chuck 536. The pressure that is provided under the edge periphery of wafer 508 is distributed such that a bearing surface is formed in gap Z1 568, and may be in the range of approximately 0.3 MegaPascal (MPa) and approximately 0.6 MPa. It should be appreciated, however, that the air or water pressure under the edge periphery of wafer 508 may vary.

In the described embodiment, outside edge surface 550 does not extend to the edge of wafer 508. That is, wafer 508 is positioned such that the outside edge of wafer 508 is not supported on a bearing surface or a hydrodynamic support surface. Although the distance X1 570 between the outside edge of wafer 508 and outside edge surface 550, relative to an x-direction 556 may vary, distance X1 570 is generally at least approximately one millimeter (mm).

Wafer chuck 536 may include at least one vacuum port 560 through which vacuum pressure may be applied to secure wafer 508. Also included in wafer chuck 536 is a plurality of pin chuck vacuum lands 564 that provide a solid reference surface for wafer 508 to be held against wafer chuck 536 by the force resulting from vacuum applied under wafer 508.

As shown in FIG. 5C, vacuum and polishing pad pressure 580 is distributed over portions of wafer 508 that are positioned over lands 558, 564 and opening 560 in a negative z-direction 554. Substantially only polishing pad pressure 582 is applied on wafer 508 over the edge periphery of wafer 508 in the negative z-direction 554. Vacuum pressure 586 is provided through opening 560. An air or fluid pressure is provided through opening 540 such that air pressure 584 that is distributed around the bottom edge periphery of wafer 508 over outside edge surface 550 provides support to wafer 508. Air pressure 584 may be adjusted or otherwise controlled to compensate for polishing pad pressure 582 such that gap Z1 568 is substantially maintained, e.g., such that the deflection of the edge of wafer 508 is minimized. In other words, the material removal rate at the edge periphery of wafer 508 may be substantially controlled by controlling air pressure 584 provided in gap Z1 568.

Air or water pressure 584 may be such that the pressure distribution in gap Z1 568 is in the range of approximately 0.3 MPa and approximately 0.6 MPa, as previously mentioned. When air or water pressure 584 is in the range of approximately 0.3 MPa and approximately 0.6 MPa, polishing pad pressure 582 may be in the range of between approximately 0.1 pounds per square inch (psi) and approximately 5 psi.

With reference to FIG. 6, the steps associated with one method of performing CMP using a system which provides wafer edge pressure control will be described in accordance with an embodiment of the present invention. A process 600 of performing CMP begins at step 604 in which polishing pad and vacuum pressure are applied to a wafer. That is, vacuum pressure is applied to a wafer by a wafer chuck on which the wafer is mounted, and a polishing pad applies pressure to the wafer. Then, in step 608, air or water bearing pressure is applied around the edge of the wafer. As previously mentioned, air or a fluid such as water is provided by the wafer chuck to a back surface of the wafer near the edge periphery such that a bearing surface is effectively formed between the back surface and the wafer chuck.

After air or water bearing pressure is applied around the edge of the wafer, the wafer is polished in step 612. Polishing wafer 612 generally includes steps associated with CMP. Such steps may include, but are not limited to, applying a slurry to the wafer, and translating the wafer relative to the polishing pad such that abrasive forces are created by the motion of the wafer against the polishing pad to abrade material away from the surface of the wafer. In the described embodiment, polishing the wafer may include monitoring an amount of deflection associated with the edge of the wafer when the polishing pad moves on to or off of the wafer.

A determination is made in step 616 as to whether the amount of deflection associated with the edge of the wafer is acceptable. Such a determination may be made using processes, e.g., in-situ processes, which may allow an endpoint of a polishing process to be measured. If it is determined that the amount of deflection associated with the edge of the wafer is acceptable, the implication is that the boundary layer provided by the air or water bearing is sufficient to hold the edge up in the presence of pressure applied by the polishing pad. As such, a determination is made in step 620 regarding whether a desired amount of material has been removed from the wafer. If it is determined that a desired amount of material has not been removed from the wafer, process flow returns to step 612 in which the wafer continues to be polished.

Alternatively, it if is determined in step 620 that the desired amount of material has been removed from the wafer, the indication is that the wafer polishing process is completed. Accordingly, the polishing pad and the vacuum pressure are removed from the wafer in step 624, and the process of performing CMP is completed.

Returning to step 616, if it is determined that the amount of deflection of the edge of the wafer is not acceptable, the implication is that there is too much deflection in the edge of the wafer. In one embodiment, a deflection of approximately 10 □m may be considered to be too much deflection. When it is determined that the amount of deflection of the edge of the wafer is not acceptable, the air or wafer bearing pressure around the edge of the wafer is adjusted in step 628. Adjusting air or water bearing pressure changes the characteristics of the boundary layer which effectively holds up the edge of the wafer. That is, modulating the pressure associated with the air or water bearing allows the material removal rate profile of the wafer to essentially be controlled. In one embodiment, the pressure may be modulated such that a resultant pressure on the edge periphery of the wafer is substantially equivalent to the pressure applied on portions of the wafer that are directly supported by a wafer chuck. Controlling the deflection of the edge periphery of the wafer and, hence, the material removal rate at the edge of the wafer enables the planarity of a polished surface of the wafer to effectively be controlled. Once the air or water bearing pressure is adjusted, process flow returns to step 612 in which the wafer continues to be polished.

Referring next to FIG. 7, a CMP system with a wafer chuck which is arranged to enable an air or fluid bearing to support the edge periphery of a wafer will be described in accordance with an embodiment of the present invention. A CMP system 700 includes a wafer 708 that is arranged to be planarized using a polishing pad assembly 702 and a slurry provided by a slurry supply 715. Polishing pad assembly 702 may be arrange to rotate a polishing pad over the surface of wafer 708, and may also be arranged to translate such that the polishing pad may be transitioned across the surface of wafer 708.

Wafer 708 is supported by a wafer chuck 736 which, in one embodiment, is a vacuum chuck that has a raised outside edge surface over which a bearing surface or a boundary layer is formed. Wafer chuck 736 may be arranged to rotate, e.g., wafer chuck 736 may be coupled to or may be a part of a platen (not shown) which rotates. A vacuum supply 745 supplies a vacuum to enable most of wafer 708 to be clamped onto wafer chuck 736, while an air or fluid supply 735 supplies air or fluid that allows a bearing surface to be formed. Typically, both vacuum supply 745 and air or fluid supply 735 are controllable, i.e., the amount of vacuum supplied by vacuum supply 745 and the amount of air or fluid supplied by air or fluid supply 735 may be adjusted.

During a CMP process involving wafer 708, a polishing endpoint and an amount of deflection associated with an edge of wafer 708 may be measured by a measurement system 725. Measurement system 725 generally makes measurements associated with wafer 708, although measurement system 725 may be incorporated into substantially any component in CMP system 700. By way of example, a polishing endpoint detector (not shown) of measurement system 725 may be incorporated into polishing pad assembly 702, while a deflection detector may be incorporated into wafer chuck 736.

Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or the scope of the present invention. By way of example, the height of a gap between a wafer chuck and a bottom edge of a wafer or a substrate has been described as being approximately 10 □m. The height, however, may vary such that the height is either more than approximately 10 □m or less than approximately 10 □m. In one embodiment, a higher air or fluid pressure distributed around the bottom edge of a wafer may enable a larger gap to be maintained. Generally, the specific geometry of the gap and the wafer may determine how much of a gap may be maintained by a particular air or fluid pressure.

In lieu of modulating or altering the amount of air or fluid pressure applied under an edge periphery of a wafer to control the material removal rate at the edge periphery of the wafer, modulating the amount of vacuum pressure used to secure the wafer to a wafer chuck may also effectively allow the material removal rate at the edge periphery of the wafer to be controlled. As vacuum pressure is increased or decreased, the edge of the wafer will generally move, thereby affecting the material removal rate at the edge. Modulating the vacuum pressure essentially modulates the bending stress associated with a wafer.

The pressure applied to a backside of a wafer or substrate along a periphery of the wafer may be modulated. By way of example, the pressure applied to a backside of a wafer may be modulated such that substantially only the edge of the wafer that is directly in contact with a polishing pad has a support pressure profile that balances the applied force from the polishing pad at the surface of the wafer without departing from the spirit or the scope of the present invention.

While a wafer has generally been described as being supported indirectly using a bearing surface, it should be appreciated that substantially any substrate may be supported using a bearing surface. In other words, although the edge of a wafer is described as being supported indirectly, the present invention is not limited to the support of a wafer and may generally be applied to other substrates.

Generally, the steps associated with the various methods of using and implementing the present invention may vary widely. Steps may be added, removed, reordered, combined, and altered without departing from the spirit or the scope of the present invention. For instance, for an embodiment in which air or fluid pressure associated with a bearing that supports the edge of a wafer is substantially constant, steps associated with adjusting air or fluid pressure may be removed. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims. 

1. A chemical mechanical polishing (CMP) apparatus comprising: a wafer; a polishing pad to polish a surface of the wafer; a polishing pad structure to rotate the polishing pad over the surface of the wafer; and a wafer chuck to support the wafer, the wafer chuck being arranged to support a first portion of the wafer that is in physical contact with the wafer chuck and to indirectly support a second portion of the wafer that is not in physical contact with the wafer chuck, wherein the second portion of the wafer is supported by the wafer chuck using a bearing surface arranged between the second portion of the wafer and the wafer.
 2. The CMP apparatus of claim 1 wherein the wafer chuck is arranged to support the first portion of the wafer using a vacuum pressure.
 3. The CMP apparatus of claim 1 wherein the wafer chuck defines at least one opening, the CMP apparatus further including: a first supply, the first supply being arranged to provide the wafer chuck with a supply through the at least one opening that is arranged to create a boundary layer between an edge surface of the wafer chuck and the second portion of the wafer, wherein the boundary layer forms the bearing surface.
 4. The CMP apparatus of claim 3, wherein the first supply is an air supply, and the bearing surface is an air bearing surface.
 5. The CMP apparatus of claim 3, wherein the first supply is a fluid supply, and the bearing surface is a fluid bearing surface.
 6. The CMP apparatus of claim 1 wherein the second portion of the wafer is a bottom edge periphery of the wafer, and wherein a boundary layer is formed between the bottom edge periphery of the wafer and an outside edge surface of the wafer chuck.
 7. The CMP apparatus of claim 6 wherein a distance between the bottom edge periphery of the wafer and the outside edge surface of the wafer chuck is approximately 10 micrometers.
 8. The CMP apparatus of claim 1 wherein the bearing surface has an associated pressure, the associated pressure being arranged to support the second portion of the wafer.
 9. The CMP apparatus of claim 8 wherein the associated pressure is adjustable to adjust an amount of support provided to the second portion of the wafer.
 10. A method for controlling a removal rate of material from a top surface of a wafer during chemical mechanical polishing (CMP), the wafer being supported on a wafer chuck such that a portion of the wafer is in direct contact with the wafer chuck, the method comprising: applying a polishing pressure to the top surface of the wafer; and applying a support pressure to an edge periphery of a bottom surface of the wafer, wherein the support pressure is applied through a gap between an edge surface of the wafer chuck and the edge periphery of the bottom surface of the wafer, the edge periphery of the bottom surface of the wafer not being in direct contact with the wafer chuck.
 11. A method for controlling a removal rate of material from a top surface of a wafer during chemical mechanical polishing (CMP) the wafer being supported on a wafer chuck the method comprising: applying a polishing pressure to the top surface of the wafer; and applying a support pressure to an edge periphery of a bottom surface of the wafer, wherein the support pressure is applied through a gap between an edge surface of the wafer chuck and the edge periphery of the bottom surface of the wafer, the edge periphery of the bottom surface of the wafer not being in direct contact with the wafer chuck wherein the support pressure is in the range of between approximately 0.3 Mega Pascal (MPa) and approximately 0.6 MPa.
 12. The method of claim 10 wherein applying the support pressure to the edge periphery of the bottom surface of the wafer includes distributing an air pressure on the edge periphery of the bottom surface of the wafer, the air pressure being arranged to act as an air bearing in the gap.
 13. The method of claim 10 wherein applying the support pressure to the edge periphery of the bottom surface of the wafer includes distributing a fluid pressure on the edge periphery of the bottom surface of the wafer, the fluid pressure being arranged to act as fluid bearing in the gap.
 14. The method of claim 10 further including: determining if the removal rate of material from the top surface of the wafer over the edge periphery is acceptable; and adjusting the support pressure to the edge periphery of the bottom surface of the wafer if it is determined that the removal rate of material from the top surface of the wafer over the edge periphery is not acceptable.
 15. The method of claim 14 wherein adjusting the support pressure includes one selected from the group including increasing the support pressure and decreasing the support pressure.
 16. The method of claim 14 wherein determining if the removal rate of material from the top surface of the wafer over the edge periphery is acceptable includes identifying an amount of deflection associated with the edge periphery of the bottom surface of the wafer.
 17. A method for controlling a removal rate of material from a top surface of a wafer during chemical mechanical polishing (CMP), the wafer being supported on a wafer chuck the method comprising: applying a polishing pressure to the top surface of the wafer; applying a support pressure to an edge periphery of a bottom surface of the wafer, wherein the support pressure is applied through a gap between an edge surface of the wafer chuck and the edge periphery of the bottom surface of the wafer, the edge periphery of the bottom surface of the wafer not being in direct contact with the wafer chuck applying a vacuum pressure to the bottom surface of the wafer, the vacuum pressure being arranged to secure a portion of the bottom surface of the wafer that is not the edge periphery against the wafer chuck; and adjusting the vacuum pressure to control the removal rate of material from the top surface of the wafer over the edge periphery.
 18. An apparatus for controlling a removal rate of material from a top surface of a wafer supported on a wafer chuck during chemical mechanical polishing (CMP), the wafer being supported on the wafer chuck such that a portion of the wafer is in direct contact with the wafer chuck the apparatus comprising: means for applying a polishing pressure to the top surface of the wafer; and means applying a support pressure to an edge periphery of a bottom surface of the wafer, wherein the support pressure is applied through a gap between an edge surface of the wafer chuck and the edge periphery of the bottom surface of the wafer, the edge periphery of the bottom surface of the wafer not being in physical contact with the wafer chuck.
 19. The apparatus of claim 18 wherein the means for applying the support pressure to the edge periphery of the bottom surface of the wafer includes means for distributing an air pressure on the edge periphery of the bottom surface of the wafer, the air pressure being arranged to act as an air bearing in the gap.
 20. The apparatus of claim 18 wherein the means for applying the support pressure to the edge periphery of the bottom surface of the wafer includes means for distributing a fluid pressure on the edge periphery of the bottom surface of the wafer, the fluid pressure being arranged to act as fluid bearing in the gap. 